In the aircraft industry there are a myriad of situations where hydraulic power is called upon in airplanes to effect the actuation of a component. Many aircraft have flight control surfaces, such as flaps or a horizontal stabilizer surface, that are moved and positioned by an aircraft actuation system incorporating a hydraulic motor of the axial piston type. The hydraulic motor has a variably positionable wobbler for controlling displacement of the motor. A control structure is provided for causing fluid flow through the motor. The motor includes a rotatable cylinder block with a series of cylinders, and respective pistons, arranged in a circular array for rotation with the cylinder block. The control structure further includes a piston connected to the wobbler for setting the motor at either minimum or maximum conditions or any condition therebetween. A servo valve also is provided for setting the position of the piston in the control system. The piston is responsive to the pressure drop across the motor in either direction of operation to set the motor displacement at the least value possible for the load condition on the motor.
An example of hydraulic motors having variably positionable wobblers for controlling displacement of the motor are shown in my U.S. Pat. No. 4,487,109, dated Dec. 11, 1984, which is assigned to the assignee of the present invention.
For primary flight control surfaces in aircraft, energy efficient power drive units of the character described above conserve hydraulic power. Hydraulic flow requirements generally are determined by the design point. This is true when the ratio of no-load rate to design point rate is approximately 4:1 or less. This invention addresses conservation of flow at low-load when the low-load rate to design point rate is greater than 4:1. In other words, hydromechanical actuation systems for primary flight control surfaces have two operating modes, namely: a high load/low rate (design point) and a low-load/high rate. At low-load operation, the wobbler will seek a minimum displacement to maintain continued operation. For an application where the ratio of no-load rate to design point rate is greater than 4, the motor wobbler displacement required may be so low (i.e. less than 1.5.degree.) that the unit will not start or continue to run. The present invention addresses this problematic condition by employing an adjustable port plate combined with the variable displacement wobbler, as described in greater detail hereinafter.